That doesn't add complexity. One thing is the most valuable to do right now and you only get it if you finish making it, so it is correct to spend all the production on one thing if the alternative is just getting that thing slower and the other thing no faster than otherwise.
For this to be valid as a strategy game, the multiple production channels must not be allowed to work on the same thing. As a corollary of that requirement, there must be little freedom to slide productivity from one channel into the others.
So, that is, the multiple channels have their own output rating and operate separate from each other. Personally I don't see what it adds, given that now you're just making a choice of what is the second most important thing to make after deciding on the first one. If you're allowed to pick up the second project with the prime productivity when the prime project is finished. If you're not allowed to do that, then you'll also be hounded by wondering if it's better to spend the subprime production on a -third- most important thing instead of second so that the prime productivity can shift to the second thing and finish it faster, in case the two decisions are out of sync.
Another option is if the option to split production increases the total amount of hammers outgoing from the city, but still decreases the highest quality productivity being made. But I just boggle at why I would want to deal with this.
It shouldn't be available to the Player to work on the same thing in all the Production Channels, and the production slider and the ability to just choose one build channel instead of multiple is the fix to that (it's a choice, so if you want to, you can just use just one Channel like always, and queue items for that one channel).
But the Multiple Channels don't actually have their own Output, but draw the Production from the City's total Production Yield. It's the Modifiers and Effects that affect the Output of each Channel based on each one's Production Item.
True, there won't be any (or much) difference in efficiency between a Single Channel and a Multi Channel Prod System, but the main Idea behind this System is that it would allow for more varierty in the Production Process. It's mostly a Flavor thing for People who love to micromanage Cities, like myself, and min-max not just the Yields, but I can also see this Mechanism helping with many other Things in the Game, but for that, we have to take out the Civ Glasses:
1) Projects: A City can build something while it's working on a Project that generates more of a Yield/Resource per Turn while consuming something else (Like Gold from a Commerce Project - so that you can get that Gold to finance the Unit that gets built).
2) Cross-City Projects: Cities could help each other to build a Wonder for example, without having to put all production resources into that Project.
And the Combo between 1 and 2 would allow for:
3) If we would have a more Complex Resource System, we could have Cities that can specialize on something, for example, Units that require multiple Resources that need to get produced first in some Buildings:
- We could activate a City Project of a Building that produces a certain amount of Swords by consuming Iron. The City can build anything while this Project is running. Reason: The City will only produce Swords and consume Iron while you're telling it to, so you can only produce the Amount of Swords you want it to, so you don't lose Iron that you wanted to also use to build Armors for a Knight Unit or lose it because of a limited resource Cap that you can have at a Time (Things getting rusty?).
- If we include Modifiers and Effects (from Buildings, Policies...etc) that apply to certain Cities or certain Production items, this would make Cross-City Projects more important. Example:
- a City that has modifiers that decrease the production cost of an Ironclad Unit and give Abilities/XP/Promotion to that Unit, can focus on training the Units for the Ironclad, while another City is building the Ships from Iron, that are required for the Ironclad Unit to be finished.
- This way, we can make Cities specialize on something(s). a City can specialize on the Training of Sea Military Units, another on building Ships (from any Era). (The Mutually exclusive Buildings would play out here). This would fix the Issue where a Coastal City can freely train Cavalery Units, which doesn't make sense. at least they should be weaker than the ones from a City with a Horses resource.
All of that would add to a more alive Economy System and improve the Empire Management Aspect of the Game. So that each City can specialize on something and the Cost of that would be a more strategic War System: If you want to cripple a Military Power House, then cut the Resource Stream that maintains that by conquering the Cities that produce those Resources and use them to your Advantage (Especially if the Resources get stored in Cities and aren't available to the Player at any Time, something that bothers me in Civ, where conquering a City doesn't even give Gold).